A substantial amount of research and development is being done to reduce our dependency on petroleum-based energy and to move us toward more sustainable energy sources, such as wind energy, solar energy, and biomass. Of these three sustainable energy sources, biomass has received significant attention. One reason for this is because biomass is widely available in many forms. Also, the economy of agricultural regions growing crops can greatly benefit from biomass to transportation fuel plants. Thus, governmental bodies in agricultural regions are very supportive of proposed biomass plants. In order to convert biomass to high value products, such as transportation fuels, one promising process is to first convert the biomass to syngas by gasification. Gasification is a well-known process for producing synthesis gas (syngas), which is a gas mixture containing varying amounts of carbon monoxide and hydrogen as the major components.
Various types of gasifier designs are known. The most common type of gasifier used in biomass gasification is believed to be an up-draft fixed-bed design (counter-current) design, in which air, oxygen and/or steam flows upward through a permeable bed of biomass and counter-currently to the flow of ash and other byproducts of the reaction. Typical up-draft gasifiers have significant technical shortcomings. First, the introduction of air into the hot gasification chamber partly combusts the biomass, resulting in a lower overall heating value compared to gasifiers that employ indirect heating. Second, if air is used as the gasification agent, nitrogen in the air which is a diluen, reduces the energy content per unit volume of output gas, making the output gas less useful for such things as gas turbines, for storage, and for subsequent chemical processing. Third, tars and phenolic hydrocarbons produced in an up-draft gasifier require removal to reduce emissions, avoid fouling of a gas turbine, and avoid catalyst poisoning when used to create liquid fuels. The removal equipment adds to system complexity and size, with the result that for economic reasons the gasifier is usually limited only to large installations. Because biomass is a low-energy content fuel and is dispersed geographically, a large-scale gasifier requires transport and storage of the biomass, which negatively affects the economic payback for the system.
A more advantageous type of gasifier for biomass is a fluidized bed gasifier whereby a volume of gas, such as steam and air and/or oxygen is passed through a bed of biomass with sufficient velocity to create a fluidized-bed of biomass particles. This mode of operation is advantageous to up-draft fixed bed gasifiers in that the fluidized bed allows for a more uniform temperature distribution within the gasifier and thus can result in a higher syngas yield and a reduction in unwanted by-products, such as tar and soot. This type of gasifier suffers from a requirement that the particles of biomass must be reduced in size by several orders of magnitude that can consume a significant amount of energy—in some cases up to 30% of the energy contained in the biomass itself.
In view of the above, there is a need for biomass gasification processes and equipment that are economically practical for use at medium- to small-scale installations, including direct sources of biomass such as agricultural operations (for example, farms), factories in which biomass materials are starting materials and/or byproducts (for example, paper mills, ethanol plants, etc.), bioplants, and small towns or villages. There is also a need for biomass pretreatment processes that result in a more efficient and economical feed for gasification. One such pretreatment is torrefaction. Torrefaction is a mild pretreatment of biomass at a temperature from about 200° C. to about 350° C. The properties of the biomass are changed to increase its heating value, reduce its tendency to degrade during storage, and make it more friable and hence easier to mill. Conventional torrefaction processes are used to produce a densified product that can be used in place of or in conjunction with coal.
While both torrefaction and gasification of biomass are well known, there is still a need in the art for improved processes combining these two technologies that can lead to a more efficient and economical process for converting biomass to transportation fuels.